Electrostatic spray gun



E. KOCK ELECTROSTATIC SPRAY GUN Oct. 7, 1969 Filed Jan. 1o, 1968.""Qsaen has!! MT A DHH N w wm. NW Wlnuhul 11km uuwlrwlh www www, .lllllHw., .LIN MIM. @www x In- .www

QM nlm @VMTN United States Patent O 3,471,089 ELECTROSTATIC SPRAY GUNErhard Kock, Toledo, Ohio, assignor to The De Vilbiss Company, Toledo,Ohio, a corporation of Ohio Filed Jan. 10, 1968, Ser. No. 696,938 Int.Cl. Bb 5/02 U.S. Cl. 239-15 6 Claims ABSTRACT OF THE DISCLOSURE Theinvention is directed to an air atomizing electrostatic spray gun havinga spray head of insulating material. Electrostatic charging of the lluidstream is accomplished by conduction, and it has been found thatprolonged spraying causes a charge to accumulate on the spray head, witha resulting deterioration in transfer efficiency. The invention includesmeans to maintain the efficiency of the system by the incorporation of agrounded electrode to remove any accumulation of charge from the frontsurfaces of the gun.

BACKGROUND AND BRIEF DESCRIPTION OE THE INVENTION It has long been knownthat an electrostatic charge can be imparted to sprayed paint particlesby conduction from an electrode recessed within the gun. Preferably, thedistance to the electrode from the front of the gun is in excess of thedistance that can be referred to as the sparking distance in air. Suchguns comprise a spray head made of insulating material such as glass orplastic. Being held by a grounded handle or support the surface of thegun body, then, is subject to a potential difference from the dischargeend where the potential is at or near the potential of the chargedmaterial stream, to ground potential at the support. It has recentlybeen discovered that it is desirable in the interests of high transfereciency to avoid building up a charge or potential on the surface of thespray head which might distort an electrostatic field between the sprayhead and the grounded work being coated. In contradistinction to knownspray guns in which the coating material stream is charged by ionbombardment, conduction charging is accomplished by transfer ofpotential directly from the recessed electrode to the fluid coatingmaterial stream and there is no Well defined point of origin for anelectrostatic field between the spray head and the grounded work.

I have found that an electrostatic spray gun having a plastic spray headand in which the coating material stream is charged by conduction has arelatively high transfer eiciency when spraying rst commences, butdeteriorates in efficiency after spraying persists. The time requiredfor efficiency deterioration depends on many factors which are notpresently fully understood and may require from several seconds toseveral minutes. The full efficiency of the gun can be restored byperiodically grounding the spray head or sometimes by periodicallytriggering the gun on and olf. I have now found that the efliciency canbe substantially maintained during even the most prolonged spraying if agrounded path is established over which the surface charge, if oneexists, may be dissipated continuously during operation of the gun.

Since it is ideally necessary for the front elements of the spray gun tobe made of a plastic material, and since it is also highly desirablethat the Huid control needle for the spray gun be located close to thefront thereof to prevent an excessively long column of liquid fromstanding in the gun when it is turned off, I have found it to beimportant that the uid control needle for the spray gun be made of aplastic material and that the valve seat with ice which the needlecooperates should also be made of a plastic material and, therefore,easily replaceable from the front of the gun.

It the spray gun is of the type that utilizes air horns to produce afan-shaped pattern, I have found that regulation- BRIEF DESCRIPTION 0FTHE DRAWINGS A preferred form of the invention is shown in the attacheddrawings, in which:

FIG. l is a longitudinal sectional view, with parts in full, and otherparts diagrammatically shown, of an electrostatic spray gun embodyingthe present invention;

FIG. 2 is a section on line 2-2 of FIG. l, somewhat enlarged;

FIG. 3 is a section on line 3 3 of FIG. 1, somewhat enlarged; and

FIG. 4 is a fragmentary section on line 4-4 of FIG. 2.

One form of an electrostatic spray gun incorporating the presentinvention, as shown in the drawings, includes a metallic electricityconducting, grounded, handle and body portions 10 into which fluidcoating material is fed by a fluid hose 11. Air for atomizing and, ifdesired, shaping a spray of coating material enters the handle end ofthe gun through an air hose 12 from any convenient air source 13. Theair hose 12 is woven with a conventional grounded strand in its wall sothat the gun body and handle may be grounded in a known manner. Aninsulated charging wire or cable 14 is taken from a power source 1Sinside the air hose 12 to connect to the electrostatic charging systemhereinafter described.

Within the handle portion of the spray gun the air is passed through aseries of radial passages 16 in a iitting 17 and thence through anopening 18 into the normal gun air passage 19 which is controlled by aconventional air valve 20 operated by a trigger 21.

At the front of the metallic body portion 10 the spray gun includes abarrel 30 of insulating material held against the body by a gland nut31. A spray head 32 of insulating material consisting of an air cap 33and a liuid tip 34 is attached to the end of the barrel 30 by aninsulating gland nut 35. In one form of spray gun the air cap isprovided with air horns 36. Details of the air cap and uid tip will besubsequently disclosed.

The charging wire'or cable 14 passes through the normal air passage 19of the spray gun into the insulating barrel portion and through an axialbore extending part- Way through the barrel where the cable contacts acharging electrode 40 which extends radially of the barrel into acoating material passage 41 which extends through the barrel 30 andreceives coating material from the uid hose 11. An elongated needlevalve 43 controls the discharge of coating material from the passage 41and the needle valve is operated by trigger 21 in the normal manner. Theneedle valve seats against a valve seat 44 formed in a plastic materialelement 45 that is threaded into the front of the barrel 30. The frontportion of the needle valve, at least that portion thereof within thebarrel 30, is made of a plastic material so that the plastic needlevalve 43 seats against the plastic seat 44. Fluid coating material inthe passage 41 is charged by conductivity from the charging electrode40, and the electrode 40 is spaced rearwardly from the front of the gunby a distance greater than the sparking distance in air. The electrode40 is charged to a voltage that is lower than the voltages used withpresent electrostatic spray guns in which charging of the sprayparticles takes place by ion bombardment. In such known guns thecharging voltage must be high enough (at least 40 kv.) to causesubstantial ionization of air in the vicinity of the charging electrodewhich is usually made as a sharp point. With conduction charging as inthe present spray gun there is no measurable air ionization.

The fluid tip 34 comprises an insulating body that is threaded to theend of the insulating barrel 30. Fluid passing from the fluid needlevalve flows through the interior of the tip 34 to a discharge orifice 50which may be a single axial hole or may be an annular orifice as isknown from the copending application of Juvinall and Kock Ser. No.677,981.

The fluid tip is formed with locating and centering ribs 51 that engagethe interior surfaces of the air cap 32 to maintain the proper locationof these cooperating parts.

The air cap 32 is an insulating body formed with a central air orifice53 which cooperates with the exterior of the fluid tip 34 to form anannular discharge passage for the atomizing air.. Atomizing air is takenfrom space 23 at the front of the metallic gun body 10 through alongitudinal bore or passage 55 (FIGS. l and 3) to an annular space 56within the gland nut 34 and at the front of the insulating barrel 30.The air thus flows from the annular space 56 to the annular atomizingair orifice 53.

The air cap is located circumferentially relative to the barrel 30 by aplurality of locating pins 57 which extend into holes 58 in the air capand into holes 59 in the barrel (FIG. 4). At least one of the locatingpins 57, in accordance with the present invention, is made of metal andis made to perform an additional function. The metal pin 57 is grounded,through an elongated spring 60, back to the gun body through a barrellocating pin 61. It will be apparent that other metal elements may beused as a grounding electrode which perform no locating function.

It has been found that an electrostatic spray gun in which thedischarged uid is charged by conduction as in the present instance bythe electrode 40 will exhibit a satisfactorily high transfer efficiencywhen spraying first commences, but that the transfer eiciency falls offafter prolonged spraying. Transfer efficiency is a ratio of paintsprayed to paint actually deposited on the work and should obviously bekept as high as possible. The time required for deterioration may varyfrom a few seconds to several minutes. It has also been found that theinitial high efficiency can be restored if the spray head (the air cap,the fluid tip and the gland nut surfaces) are periodically grounded asby touching the exterior of the spray head with a grounded wand. Attimes, even triggering the spray gun on and off seems to have a similareffect. The precise reason for this phenomenon is not fully understoodbut it is believed to be attributable to the accumulation of a potentialor charge on the exterior of the spray head which serves to distort ordiminish the electrostatic field between the spray gun and the work.With conduction charging of the paint stream there is no well definedelectrode for the origin of the electrostatic field as is the case wherethe paint is charged by ion bombardment. It has been determined that thepresence of a grounded electrode in contact with the surface of thespray head has the effect of noticeably increasing the transferefiiciency of the spray gun, probably by dissipating the chargeaccumulated on the surface of the spray head. In the form shown, the pin57 is in contact with the surface of the spray head or air cap at pointX and, of course, is grounded through the elongated spring 60 and thebarrel locating pin 61 to the gun body 10. It will be seen that theelectrode is protected by the volume insulation of the air cap 32 and ofthe barrel 30 so that there is no current ow between the charged paintstream and the grounded electrode. It is believed that any charge thataccumulates on the surface of the air cap will flow over the surfaceunder the gland nut 35 to grounded point X, and, any charge thataccumulates on the exterior of the fluid tip has a similar leakage 4path across the exterior surface of the fluid tip to the interiorsurface of the air cap by reason of the mating relationship between theinterior of the air cap and the ribs 51, thence along the remaininginterior surface of the air cap to the grounded point X.

The phenomenon of charge accumulation, if this be the reason foreficiency deterioration, appears to be related not only to the surfaceconfiguration of the front of the spray head, but also to the surfaceand volume characteristics of the material from which the spray head ismade.

In one spray gun according to the present invention, the air cap isprovided with air horns 33 which serve to discharge shaping air jetsagainst the atomized fluid stream so that the pattern of sprayedparticles is elongated or fan-shaped. Air horns for similar purposesare, of course, well known in the art. In conventional air atomizingspray guns the air horns protrude beyond the front of the gun. In thepresent instance the air horns are rounded off to prevent fieldconcentration and, for the same purpose, terminate about in a planepassing radially across the atomizing air orifice 53. As shown in FIGS.2 and 3 the insulating barrel 30 is provided with two angularly spacedaxial air passages 55 and 75 from air space 23 at the front of the gunbody 10 to the annular space 56 within gland nut 35.

To separate the air in this latter annular space there is a readilycompressible seal 65 within the annulus which is compressed axiallybetween the rear face of the air cap 33 and the front of the barrel 30.The seal 65 divides the annulus 56 into two radially spaced chamberswith the inner chamber 56a receiving atomizing air from barrel passage55 while the outer chamber 56b receives shaping air from barrel passage75. The passage 75 is provided with an adjustable valve 76 intermediateits length, and valve 76 is operated manually by a knob 77 extendingradially from the barrel. By this expedient the air flowing to the outerchamber 56 is adjustable independently of the quantity of air to theinner chamber 56a which is used for atomization. From the outer chamber56b the air is led into the axially and inwardly directed passages inthe air horns 33, said passages being designated 78 in FIG. 1. Byadjusting the position of valve 76 through knob 77 the operator canelongate the pattern to any desired extent or, by shutting off theshaping air can utilize the spray gun with a round pattern which is, insome instances, advantageous when spraying highly open articles such asbicycle frames or tubular metal furniture.

While the invention has been disclosed in conjunction with a specificform and disposition of the parts, it should be expressly understoodthat numerous modifications and changes therein may be made.

What I claim is:

1. In an electrostatic spray gun having a barrel and spray head ofinsulating material in which a stream of fluid material is charged byconduction from an electrode recessed in the barrel and in which thecoating material is atomized by interaction with air, the improvementcomprising a grounded electrode recessed within the spray head, saidelectrode being insulated from the charged uid material stream by theinsulation of the spray head and in direct contact with the surface ofthe spray head to dissipateI any undesirable charge accumulation on suchsurface.

2. The improvement defined in claim 1 in which the spray gun has agrounded metallic body portion and an insulating barrel, and saidelectrode extends axially through said barrel to said grounded bodyportion.

3. In an electrostatic spray gun, a metallic handle and -body portion,an insulating barrel carried by and extending forwardly from said bodyportion, said barrel having axial passages therethrough for the passage'of air and fiuid coating materials, means to supply air and fluidcoating materials respectively to said passages, a valve seat ofinsulating material received in said barrel at the front thereof and atthe front of said fluid coating material passage, said valve seat beingremovable from the front of said barrel, a needle valve disposed in saidcoating material passage and operable by a trigger carried by said bodyportion, at least that portion of said needle valve within said barrelportion being of insulating material and cooperating with saidinsulating valve seat to control the fiow of coating material from saidpassage, a fluid tip of insulating material threaded to the end of saidbarrel and having a bore therethrough to receive coating material fromsaid valve seat and terminating in a forward discharge orifice, an aircap of insulating material carried by said barrel portion in surroundingrelationship to said fiuid tip, said air cap having an annular airorifice surrounding said uid orifice and receiving air from said barrelair passage to atomize coating material discharged from said dischargeorifice, a charging electrode carried in said insulating barrel andhaving a portion thereof extending into said fluid coating materialpassage rearwardly of said fluid orifice, and means to impose a voltageon said electrode sufficient to cause atomized particles discharged fromsaid discharge orifice to carry a significant electrostatic charge.

4. The combination of elements defined in claim 3 in which said barrelhas two axial passages for air, one of said passages carrying airdirected to said annular air orifice and the other of said passagescarrying shaping air, air horns carried by said air cap to receive saidshaping air and to discharge it against a stream of atomized particles,and means to control separately, the air owing in said atomizing airpassage and said shaping air passage.

5. The combination of elements defined in claim 4 in which said airhorns terminate in a plane that is not be'- yond a plane passingradially across said atomizing air orifice.

6. The combination of elements defined in claim 3 and a groundedelectrode recessed within said barrel, said electrode being insulatedfrom said charged fluid material passage and in direct contact =witl1the surface of said air cap, said electrodel extending through saidbarrel to grounding contact with said metallic body portion, whereby asurface charge built up on said spray head will be dissipated to saidbody portion.

